1. Field of the Invention
The present invention relates to a color sensing method and a device therefor.
2. Description of the Prior Art
Many color sensing devices have been developed. For example, a color sensor shown in FIG. 1 (a) consists of a p-type layer 1, an n-type layer 2 and another p-type layer 3, each layers being formed successively in a mono-crystalline silicon material. The color sensor has two p-n junctions connected reversibly as indicated clearly by an equivalent circuit thereof shown in FIG. 1 (b), wherein PD1 and PD2 denote photodiodes with p-n junctions at a shallower position and at a deeper position, respectively. As shown in FIG. 2 ,the photodiode PD1 is sensitive to blue light while the photodiode PD2 has a peak of sensitivity at an infrared light. A color of an incident light can be determined from a ratio of the sensitivities of both PD1 and PD2 measured with respect to the wavelength of incident light.
It is necessary to add an infrared-absorbing filter in order to adapt the color sensor only to a visible light. However, this makes the structure thereof complicated. FIG. 3 shows the characteristics of the sensitivities of the photoconductors PD1 and PD2 when an infrared-absorbing filter is added.
FIG. 4 shows another prior art color sensor made of amorphous silicon (hereinafter referred to as a-Si) in combination with three-color separation filters (See the papers of Nakano et al., the Proceeding of the 43-rd meeting of the Japanese Society of Applied Physics, P.318). The color sensor is made by depositing a transparent electrode layer 12 and an a-Si layer (p-i-n) 13 successively on a glass layer 11, by forming three rear side electrodes 14, 15, 16 each in correspondence to each of three colors (red, green and blue), and by fixing filters 17, 18, 19 for transmitting one of these three colors respectively on the light-entering side (the lower side) of the glass plate 11 opposite to the electrodes 14, 15, 16, respectively. The incident light is decomposed into three colors by the three-color separation filters, and each separated color is transformed into electric signals in the a-Si layer 13. The color of the incident light can be identified from these electric signals. However, the structure of color sensor of this type becomes complicated, and, also, its cost becomes high.
An a-Si image sensor of a contact-type as shown in FIG. 5 (Jap. Patent laid open publication No. 99863/1984) can identify the color of incident light without any color filter. The image sensor is made by forming a plurality of separate metallic electrodes 22, a non-doped a-Si layer 23 and an transparent electrode layer 24 successively on a substrate 21.
The image sensor can identify the color of incident light by utilizing such a phenomenon that the spectral sensitivity varies with the electric voltage applied on the a-Si light-receiving surface. A ratio of two photo currents obtained, when two different bias voltages (for example, zero and a predetermined bias value) are applied, is calculated. The relation between the ratio and the wavelength of light has been obtained already. Therefore, the color of the incident light can be decided from the ratio obtained with respect to the incident light by referring to said relation.
Because the photoelectric part of the image sensor is made of a non-doped a-Si film, the variation of the photocurrent due to the bias voltage is small so that it is hard to identify the color according to the predetermined relationship obtained. Further, because the transparent electrode 24 is not separated into elements, different bias voltages cannot be applied at the same time so that the color cannot be read in real time.